What does service engine soon mean on a 2002 Altima 2.5S?

Answer 1

Any time the SES, MIL, CEL(Service Engine Soon, Maintenance Indicator Light, Check Engine Light) comes on, it is due to the onboard computer detecting a condition through its sensors that indicates the need for attention. Usually this is in response to either a preset condition which is in computer memory(like a mileage-related maintenance need); or, more commonly, something is out-of-range in its input to the computer. When this occurs, the computer sets a 'code' in its memory related to the 'fault' detected, and turns the light on. This is a signal to the operator that a code is present which must be retrieved by a technician in possession of the appropriate scan-tool for the purpose of determining what may be wrong. Sometimes, the computer indicates something which can be misinterpreted, like 'O2 SENSOR OUT-OF-RANGE'. This means just exactly what it says: The computer has, for some reason, detected in reading from the O2 sensor input line a signal which is inappropriate as a response for the conditions as they existed when the code was set. It does NOT mean that the O2 sensor is bad, or in need of replacement. It is the mechanic's job to find the reason for the computer to have responded in the way that it did. Perhaps a dirty air filter caused this. Maybe there was a break in the signal wire. Maybe an exhaust leak has allowed outside air into the exhaust stream which was being measured by the sensor, causing an inappropriate reading to be obtained relative to engine speed. Maybe the coolant temperature sensor went bad, making the computer believe the engine was cold when it was hot, and a reading of the O2 sensor output didn't fit in with this information as a proper response. In each of these cases, replacement of the indicated sensor would serve to do nothing except to confuse the issue. That's just an example of why the services of a professional technician are required for the diagnosis and repair of today's computerized vehicles. The alternative is a needless waste of money and time, accompanied by an intolerable rise in frustration. Have the codes read, and go from there.